MOTO SHIFTER KART ENGINE OVERVIEW

• 1999 CR125 motocross engine with 1997 gear set (6 speeds)
• Designed for motocross wide power band for use by general public
• Rugged clutch
• Originally used pump gasoline.  Requires 100+ octane for shifter kart application.
• Long parts life

FITTING ENGINE TO KART

• Steel is more rugged but heavier
• Front horizontal mounting bolt size 3/8” or 10mm
• Flex heel mount must be checked regularly for tightness
• Clearance required for movement to adjust chain tension
• Support air filter to prevent carb fall out

• Provide clearance for shift linkage when seat flexes
• J-arm to seat strut clearance for complete gear shifts
• J-arm tight on splines to eliminate play
• Check front to rear weight bias

• Engine package is tuned to a specific pipe and silencer
• Strive for leak free exhaust joints
• Adjust pipe and silencer position to eliminate binding
• Use springs with adequate tension to eliminate pipe push-off
• Lightly safety wire springs to dampen vibration
• Insert EGT probe or plug EGT hole in pipe
• Route EGT probe wiring on top of frame rails
• Mount silencer to engine to allow it to move with pipe

• Carbs intended for motorcycle use (lean into turns)
• Shifter karts don’t lean (fuel sloshes to sides of bowl in turns)
• Dual pump systems keep fuel level constant over main jet

• Use ¼” id lines minimum
• Route lines so they are not pinched when seat or kart flexes
• Fuel return and vent fitting on fuel tank should be 3/16” id minimum
• Inadequate tank vent or fuel return causes rich bog in turns
• Zip tie all fuel fittings
• Pulse fitting on pumps should point down to allow oil drain back
• Mount pump above pulse fitting on engine to allow oil drain back

• Allow 3/8” play on clutch handle
• Limit travel to one inch once the clutch is engaged
• Route clutch cable with gradual bends

• Use kart pedal stop to adjust full throttle not carburetor arm tension

COOLING SYSTEM

• Transfer heat from engine to air
• Use 5/8” id hose minimum to and from engine and radiator
• Large thin single core radiator transfers most heat to air
• Maximum heat transfer occurs with radiator at or near vertical.
• A 1” lip around radiator usually lowers water temp 5 degrees F.

• Use distilled water to prevent deposits in engine and radiator
• Use Redline water wetter or equivalent to prevent corrosion in cooling system
• Corrosion of water pump shaft requires replacement of expensive part
• Anti freeze works but is not legal for racing

SETTINGS AND TUNING RECOMMENDATIONS

• Lap times (lower is better)
• Speed at end of longest straight
• RPM at end of longest straight
• EGT at end of longest straight
• Throttle response from idle, part throttle in turns and during gear shifts
• RPM and gear when exiting turns

• Set carb to allow engine to idle
• Air screw is fine adjustment for pilot jet (0-1/8 throttle response)
• Slide needle covers 1/8-3/4 throttle opening (throttle response)
• Main jet covers ¾-full throttle opening
• ADR change of 2.7% suggests a main jet change
• Main jet sizes are about 2.7% apart
• ADR change of 7.5% suggests a slide needle position change

• Position engine on kart to prevent shift linkage interference
• Check for seat and chassis flex interference
• Check for steering wheel interference
• Several length chains should be available
• Tighten engine sprocket and axle sprocket
• Relieve binding in exhaust system

• Clean engine with carb cleaner to prevent dirt from entering engine
• Drain water from hole on bottom of water pump (capture?)
• Remove cylinder to crankcase bolts in 2 stages diagonally
• Change piston, ring, bearing wrist pin and circlips every 12 hours
• Change transmission oil every 15 hours (Redline lite wt. Shockproof)
• Check for water and oil leaks
• Tilt kart when refilling with water to displace air in water pump

• Run one jet richer than normal
• Gradually increase rpm and load (coming off corners briskly) during 15 minute break in
• Stop engine after 7 minutes restart for 8 more minutes
• See specific engine instructions for more detailed break-in information

• Clean engine with carb cleaner
• Remove mount, carb and coil
• Pack tightly with newspaper wads on bottom and all sides
• Place written instructions in box with engine

Call 903-769-4140 or submit the contact form on the website if you have questions.